The present invention relates to a controller and control methodology for a semi-active shock absorber. More particularly, the present invention relates to a system and method of controlling the relative motion between two masses, using a suspension that includes a shock absorber or damper. The system and method can be applied to a number of types of systems such as the primary suspension on a vehicle, which isolates the mass of the chassis from the motion of the wheels as they run over rough terrain or a truck, or boat seat that is isolated from the movements of the cab or hull. The present invention has general applicability to any system that has a vibration isolation mechanism that isolates the sprung mass from movements of the unsprung mass such as engine mounts, machinery mounts or other typical applications for isolation mounts.
Suspensions and isolation mounts generally fall into one of the following categories: passive, active or semi-active. Passive mounts usually include a passive spring and passive damper and can be tuned to provide very good isolation for a given set of conditions such as fixed masses and constant frequency disturbance into the unsprung mass. However if the mass changes due to increased payload, or the input frequency changes due to a change in speed over the ground, the isolation performance is degraded and often results in very large shock loads when the system hits the ends of travel, usually referred to as xe2x80x9ctoppingxe2x80x9d or xe2x80x9cbottomingxe2x80x9d the suspension.
Active suspensions are able to provide much better isolation over a wider range of conditions than a purely passive system. They can read a variety of sensors, then process the information to provide an optimal target force between the two masses at any time, given the power limits of the actuators and support systems. In addition, they are capable of adding energy to the system whereas passive and semi-active systems can only subtract energy. Active suspensions have not gained wide acceptance due to high cost and complexity as well as the demand for high power from the vehicles prime mover. In the case of off-road vehicles with long travel suspensions moving over rough terrain, the power draw of the suspension is prohibitive and reduces the maximum acceleration of the vehicle.
Semi-active suspensions are generally less costly and complex than fully active systems while retaining most of the performance advantages. They use the passive spring from conventional suspensions and add a controllable damper as well as the sensors and microprocessor required to allow the damper force to be controlled in real time. The damper can still only subtract energy from the system, however it can provide any level of damping that is demanded by the control method, rather than being governed by the fixed velocity/force laws that are characteristic of passive dampers.
There are a number of control methods that have been developed for semi-active suspensions, starting with xe2x80x9cskyhookxe2x80x9d method described by Kamopp, et al., xe2x80x9cVibration Control Using Semi-active Force Generator,xe2x80x9d ASME Paper No. 73DET-123, May 1974, and U.S. Pat. No. 3,807,678. This method attempts to make the damper exert a force which is proportional to the absolute velocity of the sprung mass, rather than the relative velocity between the two masses. Hence the term skyhook since the mass is treated as though it is referenced to the inertial coordinate system rather than the ground. While this method can yield very good isolation over bumps that are smaller than the amount of compression travel in the system, larger bumps cause the suspension to bottom out resulting in a large shock load being transmitted into the sprung mass.
Another method has been developed to deal with the bottoming and topping problem called the xe2x80x9cend stopxe2x80x9d method. In end stop mode, the microprocessor calculates the minimum force required to decelerate the sprung mass and prevent the suspension from bottoming. While this is effective in preventing the high shock loads from being transmitted into the sprung mass, it results in excessive suspension movement over smaller bumps. This can be very disconcerting to the operator because it prevents him from having a good xe2x80x9cfeelxe2x80x9d for the behavior and handling of the vehicle.
There have also been attempts to combine several methods and assign relative weightings or develop rules that govern the use of alternate methods under certain circumstances. Most of these efforts have been aimed at isolation efficiency as the overall goal or metric of relative merit. However there are other factors that are important in suspension systems such as transient force distribution that can influence handling and vehicle control, as well as subjective factors such as operator comfort and confidence.
The present invention solves the shortcomings of the prior art with a set of rules that will result in a practical semi-active suspension control method.
In one aspect, the present invention includes a method for determining if a shock absorber system is compressing and for generating a target control signal for shock absorber system comprising two masses coupled together by a spring having a controllable valve to adjust the energy in said system. The method includes the step of determining if the spring/mass system is compressing in a z direction by determining the current velocity of the masses with respect to one another. The method also includes the step of generating an inertial endstop signal based on the relative velocity and the relative position of said masses, the inertial endstop signal is proportional to the minimum acceleration necessary for one of the masses to arrive at a position of minimum travel at approximately zero velocity. The method also includes the step of generating a damped signal based on a spring force constant, the critically damped signal is proportional to a critically damped trajectory of at least one of the masses, and generating a comfort signal defined as an upper force threshold for said critically damped signal. The method selects one of the signals as a target signal to control said valve and thereby adjust the energy in the spring/mass system.
In another aspect, the present invention includes a method for determining if a shock absorber system is expanding and for generating a target control signal for shock absorber system comprising two masses coupled together by a spring an having a controllable valve to adjust the energy in the system. The method includes the steps of determining if the spring mass system is expanding in a z direction by determining the current velocity of the masses with respect to one another; generating an inertial endstop signal based on the relative velocity of the masses, the inertial endstop signal is proportional to the minimum acceleration necessary for one of the masses to arrive at a position of maximum travel at approximately zero velocity; and generating a damped signal based on a spring force constant, the damped signal is proportional to a damped trajectory of at least one of the masses. The method also includes the steps of generating a first valve prepositioning signal proportional to the valve position that permits one of the masses to freefall away from the other mass; and generating a second valve propositioning signal proportional to the valve position that permits one of the masses to controllably expand away from the other mass. The method selects one of these signals as a target signal to control said valve and thereby adjust the energy in the spring/mass system.
In still another aspect, the present invention provides a method for generating a target inertial and non-inertial energy control signal in a spring/mass shock absorber system comprising two masses coupled together by a spring having a controllable valve to adjust the energy in said system. The method includes the steps of: generating an endstop signal based on the relative velocity and relative position of the two masses, the inertial endstop signal is proportional to the minimum acceleration necessary for one of the masses to arrive at a position of maximum or minimum travel at approximately zero velocity. The method modifies the endstop signal with a signal indicative of the absolute velocity and the absolute displacement of the masses with respect to one another. The method also determines if the endstop signal should be designated as a target control signal for the controllable valve based on the relative velocity of said masses.
In yet other aspects, the present invention provides a method for generating a target multidimensional damped energy control signal in a spring/mass shock absorber system comprising two masses coupled together by a spring having a controllable valve to adjust the energy in said system. The method includes the steps of: generating a damped signal based on a spring force constant, the damped signal is proportional to a damped trajectory of at least one of the masses in the z direction; defining a critically damped coefficient; and multiplying the damped signal by the critically damped coefficient. The method further includes the steps of calculating measuring the acceleration of at least one of said two masses in the x and/or y direction, and modifying the critically damped coefficient based on the measured acceleration of at least one of said two masses in the x and/or y direction. The method also determines if the damped signal should be designated as a target control signal for the controllable valve.
Another aspect of the present invention provides a method for generating a target direct valve control signal in a spring/mass shock absorber system comprising two masses coupled together by a spring having a controllable valve to adjust the energy in the system. The method includes the steps of generating a valve propositioning signal based on the relative position and relative velocity of the masses, the valve prepositioning signal is proportional to a predefined amount of prepositioning for the valve so that the energy of the spring assumes a predefined quantity; and determining if the valve propositioning signal should be designated as a target control signal for the controllable valve based on the relative velocity of said masses.
The present invention also provides a method for modifying a valve control signal with an acceleration hedge control signal in a spring/mass shock absorber system comprising two masses coupled together by a spring having a controllable valve to adjust the energy in said system. The method includes the steps of generating a plurality of valve control signals based on the relative velocity of the masses and generating an acceleration hedge signal proportional to the addition of the acceleration or force of a first one the masses to that of the second one of the masses to drive the average acceleration or force of the second mass to approximately equal the actual acceleration or force of the first mass. The acceleration hedge signal is added to a selected one of said valve control signals.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments, the present invention is not intended to be limited to these embodiments. It should be understood from the outset that the present invention shall make use of the terms xe2x80x9cmethodsxe2x80x9d or xe2x80x9cmodular processorsxe2x80x9d, and the such terms shall be construed broadly as encompassing one or more program processes, data structures, source code, program code, etc., and/or other stored data on one or more conventional general purpose and/or proprietary processors, that may include memory storage means (e.g. RAM, ROM) and storage devices (e.g. computer-readable memory, disk array, direct access storage). Alternatively, or additionally, such methods or modular processors may be implemented using custom and/or off-the-shelf circuit components arranged in a manner well-understood in the art to achieve the functionality stated herein.
Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: